The present invention relates generally to data communication systems and more particularly relates to a transmitter for use in a spread spectrum communication system.
The use of spread spectrum communications techniques to improve the reliability and security of communications is well known and is becoming more and more prevalent. Spread spectrum communications transmits data utilizing a spectrum bandwidth much greater than the bandwidth of the data to be transmitted. This provides for more reliable communication in the presence of high narrowband noise, spectral distortion and pulse noise, in addition to other advantages. Spread spectrum communication systems typically utilize correlation techniques to identify an incoming received signal.
Spread spectrum communications systems are commonly used in military environments to overcome high energy narrowband enemy jamming. In commercial or home environments it may be used to achieve reliable communication on noisy media such as the AC powerline. In particular, certain home electrical appliances and devices can potentially be very disruptive of communications signals placed onto the powerline. For example, electronic dimming devices can place large amounts of noise onto the powerline since these devices typically employ switching devices, such as triacs or silicon controlled rectifiers (SCRs), to control the AC waveform in implementing the dimming function.
A communication medium such as the AC powerline may be corrupted by fast fading, unpredictable amplitude and phase distortion and additive noise. In addition, communication channels may be subjected to unpredictable time varying jamming and narrowband interference. In order to transmit digital data over such channels it is preferable to use as wide a bandwidth as possible for transmission of the data. This can be achieved using spread spectrum techniques.
One common type of spread spectrum communications, called direct sequence spread spectrum, is generated by first modulating the digital data and then multiplying the result with a signal having particularly desirable spectral properties, such as a pseudo noise (PN) sequence. The PN sequence is a periodic sequence of bits having a particular period. Each bit in the sequence is termed a chip. The sequence has the property of having very low autocorrelation for delays larger than one chip. In some systems, the PN sequence is replaced by a chirp waveform. Several techniques are available for use by the transmitter in modulating the data signal, including biphase shift keying (BPSK) and continuous phase modulation (CPM) techniques. Another modulation technique is minimum shift keying (MSK), which is a known variation of CPM.
The spread spectrum receiver is required to perform synchronization that is commonly implemented using an acquisition method in combination with a tracking loop or other tracking mechanism. In a noisy unpredictable environment such as the AC powerline, the tracking loop typically fails frequently causing loss of information. Communication, systems to overcome these problems are large, complex and expensive. In addition, these systems typically succeed at transmitting only one or more bits per symbol.
Receiving and demodulating signals that have been subject to PN modulation requires that the same PN code sequence be generated in the receiver and correlated with the received signal to extract the data modulation. One type of correlation technique employs a digital matched filter to compare the received digital signal with the locally generated version of the PN code. The digital filter produces an in phase (I) signal and a quadrature (Q) signal from which a digital demodulator such as a Differential Phase Shift Keying (DPSK) demodulator can derive data values. Another function of the digital matched filter is to produce correlation measurements from which synchronization signals can be generated.
In despreading a spread spectrum signal, the receiver produces a correlation pulse in response to the received spread spectrum signal when the received spread spectrum signal matches the chip sequence to a predetermined degree. Various techniques are available for correlating the received signal with the chip sequence, including those using surface acoustic wave (SAW) correlators, tapped delay line (TDL) correlators, serial correlators, and others.
The present invention is a transmitter apparatus and method for use in a spread spectrum data communications system that utilizes the Differential Code Shift Keying (DCSK) or non-differential Code Shift Keying (CSK) modulation technique. Such communications systems are applicable to relatively noisy environments such as the AC powerline.
In a CSK transmission system, the data is transmitted in the form of time shifts between consecutive circularly rotated waveforms of length T which are referred to as spreading waveforms, i.e., spread spectrum correlator sequence waveforms. The spreading waveforms can comprise any type of waveform that has suitable autocorrelation properties. During each symbol period, referred to as a unit symbol time (UST), a plurality of bits are transmitted. The symbol period is divided into a plurality of shift indexes with each shift index representing a particular bit pattern. The information, i.e., bit pattern, is conveyed by rotating the spreading waveform by a certain amount corresponding to the data to be transmitted. The data is conveyed in the degree of rotation or circular shift applied to the spreading waveform (also referred to as a chirp) before it is transmitted.
In addition to conveying information in the shift applied to the spreading sequence, additional information can be conveyed in the sign of the symbol or more generally in its phase if there is a carrier present.
In a CSK system, the data is conveyed in the absolute shift assigned to the spreading waveform. In a DCSK system, the data is conveyed in the shift differential between consecutive symbols. The synchronization scheme of the present invention is applicable to both CSK and DCSK transmission systems.
Upon reception by the receiver, the signal is input to a matched filter having a template of the spreading waveform pattern to detect the amount of rotation (or circular shift) within the received signal for each symbol. The received data is fed into a shift register and circularly rotated, i.e., shifted. For each bit shift or rotation, the matched filter generates a correlation sum. A shift index is determined for each UST corresponding to the shift index that yields the maximum (or minimum) correlation sum. Differential shift indexes are generated by subtracting the currently received shift index from the previously received shift index. The differential shift index is then decoded to yield the originally transmitted data.
The transmitter transmits data in the form of packets to the receiver. Each packet is preceded by a preamble comprising a predetermined number of symbols. The length of the preamble can be any suitable number of symbols such that the receiver is able to synchronize with the transmitter. The preamble comprises a sequence comprising any number of zero rotated symbols (or symbols with a constant fixed rotation) followed by any number of non-zero rotated symbols having a known predetermined random shift. The rotation that is applied to each symbol is independent of the rotation applied to other symbols.
In a first embodiment, the shift-index, i.e., the data received from a host source, is used to calculate an initial index. The initial index is used in combination with a counter to address a modulation waveform sample Read Only Memory (ROM). The modulation waveform may be any suitable waveform having good correlation properties. Examples of suitable waveforms include chirp waveforms and Pseudo Noise (PN) sequence waveforms.
The waveform samples are clocked out of the sample ROM starting from a position corresponding to the initial index. This yields a rotated symbol having a degree of rotation corresponding to the data to be transmitted. The number of unique symbols determines the number of bits transmitted in each symbol. An additional bit can be encoded in the transmitted signal by setting the sign of the symbol. Each symbol can have a positive or negative sign (or phase), i.e., non-inverted or inverted. Thus, an additional bit can be transmitted in addition to the bits conveyed in the rotation of the modulation waveform. If a carrier is also transmitted then, utilizing a suitable modulation such as MPSK, more than one bit can conveyed by the phase of the carrier.
The rotated sample bit stream is then encoded by a Manchester encoder, amplified using a digital buffer, such as a CMOS gate, and filtered via a band pass filter. The BPF provides any desired spectral shaping to the output waveform. The signal is then coupled to the communications channel via suitable circuitry. In the case of an AC power line channel, a signal transformer is used coupled to a high pass filter.
In a second embodiment, a FIR filter, D/A converter and operational amplifier are used in place of the Manchester encoder. The FIR filter is adapted to band pass filter the input signal and to provide one or more notches at stop bands within the pass band. These notches correspond to special frequencies wherein transmission may not be permitted (depending on the location). The notch frequencies may correspond, for example, with one or more amateur frequency bands that lie within the pass band, e.g., 4 to 20 MHz. The D/A converter and op amp function to convert the digital output of the FIR filter to an analog signal.
The invention further discloses a method of constructing a filter having one or more notches in the frequency domain. The method comprises inverting the phase of the real frequency response at each frequency it is desired to have a notch. An inverse FFT is performed and a window function is applied to the resulting time domain representation of the frequency response. The results of the windowing function are quantized, scaled, etc. and the filter tap coefficients generated therefrom.
There is provided in accordance with the present invention a transmitter for use in a spread spectrum communications system comprising an initial index calculator operative to generate an initial index in accordance with a shift index input from a host data source, a sequence memory adapted to hold a modulation waveform sequence, a counter operative to count modulo the length of the modulation waveform sequence so as to generate a complete sequence of samples rotated by an amount in accordance with the initial index, each complete sequence of samples representing a symbol, an encoder operative to encode the rotated sequence sample stream so as to shift the spectrum thereof, an amplifier adapted to generate sufficient drive current to drive a channel over which the symbols are to be transmitted, a filter adapted to spectrally shape the signal output of the amplifier and channel coupling circuitry operative to couple the filtered signal output from the filter onto the channel media.
The modulation waveform may comprise a Pseudo Noise (PN) sequence or a chirp waveform. The initial index is calculated in accordance with the following             initial      -        ⁢    index    =                    (                              PN            ⁢                          xe2x80x83                        ⁢            sequence            ⁢                          xe2x80x83                        ⁢            length                                number            ⁢                                          xe2x80x83                            ⁢                              xe2x80x83                                      ⁢            of            ⁢                          xe2x80x83                        ⁢            possible            ⁢                          xe2x80x83                        ⁢            shifts                          )            ·              shift        -              ⁢    index  
wherein the shift index comprises the data input from the host data source. The sequence sample memory comprises a Read Only Memory (ROM) storage device and the amplifier is adapted to generate a differential output. The filter may comprise a band pass filter, e.g., symmetrical band pass filter, having a pass band from 4 to 20 MHz and having one or more notches. The channel coupling circuitry comprises a signal transformer coupled to a symmetrical high pass filter. The transmitter further comprises means for encoding a bit received from the host data source as the sign of the rotated sequence. The encoder comprises a Manchester encoder.
The amplifier comprises a non-inverting gate and an inverting gate adapted to be driven in parallel by the output of the encoder so as to generate a differential output signal or a single non-inverting gate adapted to be driven by the output of the encoder. The single non-inverting gate comprises tri-state control means. Alternatively, the amplifier comprises a single inverting gate adapted to be driven by the output of the encoder.
There is further provided in accordance with the present invention a transmitter for use in a spread spectrum communications system comprising an initial index calculator operative to generate an initial index in accordance with a shift index input from a host data source, a counter operative to count modulo the length of a modulation waveform sequence for each symbol to be transmitted, the counter beginning at a point in the sequence corresponding to the initial index, a sequence sample memory adapted to generate the modulation waveform sequence rotated by an amount in accordance with the initial index in response to the output of the counter, a first filter operative to encode the rotated sequence sample stream so as to shift the spectrum thereof, an amplifier adapted to generate sufficient drive current to drive a channel over which the symbols are to be transmitted, a second filter adapted to spectrally shape the signal output of the amplifier and channel coupling circuitry operative to couple the filtered signal output from the filter onto the channel media.
The transmitter further comprises means for encoding a bit received from the host data source as the sign of the rotated sequence. The sequence sample memory comprises a Read Only Memory (ROM) storage device. The first filter comprises a Finite Impulse Response (FIR) filter adapted to function as a band pass filter with a pass band from 4 to 20 MHz. Alternatively, the first filter comprises a Finite Impulse Response (FIR) filter band pass filter having one or more notches in its pass band. The amplifier comprises a differential amplifier adapted to output a differential output signal.
There is also provided in accordance with the present invention, in a spread spectrum communications system, a method for transmitting symbols onto a communications channel, the method comprising the steps of providing a sample memory comprising samples of a modulation waveform sequence to be transmitted each symbol period, calculating an initial index into the sequence sample memory in accordance with a shift index received a host data source, generating a sample bit stream comprising the sequence rotated in accordance with the initial index, encoding the sample bit stream output of the sample memory and coupling the encoded sample bit stream onto the communication channel.
The method further comprises means for encoding a bit received from the host data source as the sign of the rotated sequence. The method further comprises the step of amplifying the encoded sample bit stream so as to provide sufficient drive current to drive the communication channel. The method further comprises the step of spectrally shaping the encoded bit stream so as to notch out one or more frequencies.
There is still further provided in accordance with the present invention a method for constructing a filter having one or more notches in its frequency response, the method comprising the steps of determining the desired frequency response including one or more notches to be placed at one or more frequencies, inverting the phase of the frequency response at each notch frequency such that a zero crossing is generated for each notch frequency desired in the frequency response, performing an inverse FFT on the resulting frequency response, applying a windowing function to the resulting time domain representation of the frequency response and generating filter tap coefficients from the results of the windowing function.
There is also provided in accordance with the present invention a transmitter for use in a spread spectrum communications system comprising an initial index calculator operative to generate an initial index in accordance with a shift index input from a host data source, a sequence memory adapted to hold a modulation waveform sequence, a counter operative to count modulo the length of the modulation waveform sequence so as to generate a complete sequence of samples rotated by an amount in accordance with the initial index, each complete sequence of samples representing a symbol, an encoder operative to encode the rotated sequence sample stream so as to shift the spectrum thereof, a filter adapted to spectrally shape the signal output of the encoder, wherein the filter comprises one or more notches in the frequency domain with phase that alternates in sign, a zero crossing in the frequency response occurring at each desired notch frequency and channel coupling circuitry operative to couple the filtered signal output from the filter onto the channel media.
The channel coupling circuitry comprises a digital to analog (D/A) converter operative to convert the output of the filter so as to generate an analog output signal and amplifier means operative to receive the analog output signal and to generate sufficient drive current in response thereto to drive the channel over which the symbols are to be transmitted and interface circuitry adapted to couple the output of the amplifier to the channel.
There is further provided in accordance with the present invention a transmitter comprising means for generating a transmission signal in accordance with data received from a host data source, a filter adapted to spectrally shape the transmission signal, wherein the filter comprises one or more notches in the frequency domain with phase that alternates in sign, a zero crossing in the frequency response occurring at each desired notch frequency and channel coupling circuitry operative to couple the filtered signal output from the filter onto the channel media.
There is also provided in accordance with the present invention, in a spread spectrum communications system, a method for transmitting symbols onto a communications channel, the method comprising the steps of providing a sample memory comprising samples of a modulation waveform sequence to be transmitted each symbol period, calculating an initial index into the sequence sample memory in accordance with a shift index received a host data source, generating a sample bit stream comprising the sequence rotated in accordance with the initial index, encoding the sample bit stream output of the sample memory, filtering the sample bit stream output of the sample memory utilizing a filter constructed to comprise one or more notches in the frequency domain whereby the phase of the frequency response is inverted at each desired notch frequency such that a zero crossing is generated for each notch frequency desired in the frequency response and whereby a windowing function is applied to the time domain representation of the response and the results quantized to yield a plurality of filter tap coefficients and coupling the encoded sample bit stream onto the communication channel.